[Garkin]

Local function "junkJunk" must be defined before line where it is called. Fixed code:

local junker = {
    name = "junker"
}
 
local junk_types = {
    "ITEMTYPE_ADDITIVE",
    "ITEMTYPE_ALCHEMY_BASE",
    "ITEMTYPE_AVA_REPAIR",
    "ITEMTYPE_CLOTHIER_BOOSTER",
    "ITEMTYPE_CLOTHIER_MATERIAL",
    "ITEMTYPE_CLOTHIER_RAW_MATERIAL",
    "ITEMTYPE_COLLECTIBLE",
    "ITEMTYPE_CONTAINER",
    "ITEMTYPE_DISGUISE",
    "ITEMTYPE_ENCHANTING_RUNE",
    "ITEMTYPE_ENCHANTMENT_BOOSTER",
    "ITEMTYPE_FLAVORING",
    "ITEMTYPE_FOOD",
    "ITEMTYPE_GLYPH_ARMOR",
    "ITEMTYPE_GLYPH_JEWELRY",
    "ITEMTYPE_GLYPH_WEAPON",
    "ITEMTYPE_INGREDIENT",
    "ITEMTYPE_LURE",
    "ITEMTYPE_NONE",
    "ITEMTYPE_PLUG",
    "ITEMTYPE_RAW_MATERIAL",
    "ITEMTYPE_STYLE_MATERIAL",
    "ITEMTYPE_TRASH"
}
 
 
local junkarmor_types ={
    "ARMORTYPE_LIGHT",
    "ARMORTYPE_MEDIUM",
    "ARMORTYPE_NONE"
}
 
local function contains(junk_types, element)
    for _, value in pairs(junk_types) do
        if value == element then
            return true
        end
    end
    return false
end
 
local function junkJunk(eventCode,bagId,slotId,isNewItem,itemSoundCategory, updateReason)
    local armor_type = nil
    local item_type = GetItemType(bagId,slotId)
    local item_link = GetItemLink(bagId,slotId)
 
    if item_type == "ITEMTYPE_ARMOR" then
        armor_type= GetItemLinkArmorType(item_link)
    end
 
    if contains(junk_types,item_type) or contains(junkarmor_types,armor_type) then
        SetItemIsJunk(bagId,slotId)
        d("junked")
    end
end
 
local function Initialize()
    EVENT_MANAGER:RegisterForEvent(junker.name, EVENT_INVENTORY_SINGLE_SLOT_UPDATE, junkJunk)
end
 
 
local function  OnAddOnLoaded(event,addonName)
    if addonName == "junker" then
        Initialize()
    end
end
 
EVENT_MANAGER:RegisterForEvent(junker.name, EVENT_ADD_ON_LOADED, OnAddOnLoaded)

I have just changed order when functions are defined.

If you want also make this code a bit faster, do not use function "contains", but define your tables as follows and just check if table keys exists:

local junk_types = {
    "ITEMTYPE_ADDITIVE" = true,
    "ITEMTYPE_ALCHEMY_BASE" = true,
    "ITEMTYPE_AVA_REPAIR" = true,
    "ITEMTYPE_CLOTHIER_BOOSTER" = true,
    "ITEMTYPE_CLOTHIER_MATERIAL" = true,
    "ITEMTYPE_CLOTHIER_RAW_MATERIAL" = true,
    "ITEMTYPE_COLLECTIBLE" = true,
    "ITEMTYPE_CONTAINER" = true,
    "ITEMTYPE_DISGUISE" = true,
    "ITEMTYPE_ENCHANTING_RUNE" = true,
    "ITEMTYPE_ENCHANTMENT_BOOSTER" = true,
    "ITEMTYPE_FLAVORING" = true,
    "ITEMTYPE_FOOD" = true,
    "ITEMTYPE_GLYPH_ARMOR" = true,
    "ITEMTYPE_GLYPH_JEWELRY" = true,
    "ITEMTYPE_GLYPH_WEAPON" = true,
    "ITEMTYPE_INGREDIENT" = true,
    "ITEMTYPE_LURE" = true,
    "ITEMTYPE_NONE" = true,
    "ITEMTYPE_PLUG" = true,
    "ITEMTYPE_RAW_MATERIAL" = true,
    "ITEMTYPE_STYLE_MATERIAL" = true,
    "ITEMTYPE_TRASH" = true
}
 
 
local junkarmor_types ={
    "ARMORTYPE_LIGHT" = true,
    "ARMORTYPE_MEDIUM" = true,
    "ARMORTYPE_NONE" = true
}
 
--modified function without "contains"
local function junkJunk(eventCode,bagId,slotId,isNewItem,itemSoundCategory, updateReason)
    local armor_type = nil
    local item_type = GetItemType(bagId,slotId)
    local item_link = GetItemLink(bagId,slotId)
 
    if item_type == "ITEMTYPE_ARMOR" then
        armor_type= GetItemLinkArmorType(item_link)
    end
 
    if junk_types[item_type] or (armor_type and junkarmor_types[armor_type]) then --make sure that armor_type is not nil before you use it as an index of the table
        SetItemIsJunk(bagId,slotId)
        d("junked")
    end
end
 
local function Initialize()
    EVENT_MANAGER:RegisterForEvent("junker", EVENT_INVENTORY_SINGLE_SLOT_UPDATE, junkJunk)
end
 
 
local function  OnAddOnLoaded(event,addonName)
    if addonName == "junker" then
        Initialize()
    end
end
 
EVENT_MANAGER:RegisterForEvent(junker.name, EVENT_ADD_ON_LOADED, OnAddOnLoaded)